A large part of the ideas upon which this invention has been based has been described in Brazilian application No. PI 8606370 for an IP (Invention Privilege) of Dec. 22, 1986, which IP corresponds to pending U.S. application Ser. No. 07/136,324 filed on Dec. 22, 1987, and lodged by the same Applicant, and which is included herein by reference thereto, though some of the aspects that make up this present invention are quite different from those in the aforesaid former application, which fact warrants the originality hereof, and its main advantages lie in such aspects.